ADP-ribosylating bacterial exotoxins are proteins that are responsible for various severe human diseases such as cholera, pretussis, and diphtheria. Diphtheria toxin (DT) catalyzes the covalent transfer of the ADPribosyl moiety of NAD+ to a dipthamide residue in eukaryotic elongation factor 2 (eEF-2). This covalent transfer inactivates eEF-2, rendering it incapable of polypeptide chain elongation, inhibiting protein synthesis and eventually killing the target cells. The objective of this proposal is to determine the transition state structure of ADP-ribosylation of eEF-2 by diphtheria toxin by measuring kinetic isotope effects (KIEs) with radiolabeled NAD+ analogues. Transition state analysis of the reaction catalyzed by diphtheria toxin will result in a better understanding of this enzyme and should provide target structures for the design of novel transition state analogues as inhibitors.